Certain research and manufacturing processes require the use of a process chamber with high vacuum. The vacuum may be required for several reasons. In some instances, atmospheric components that could cause a chemical reaction or physical damage during the process must be removed (e.g., in vacuum melting of reactive metals such as titanium). In other instances, vacuum is used to disturb an equilibrium condition existing at normal room conditions, such as in removing volatile liquid or occluded or dissolved gas from the bulk of material (e.g., degassing oils, freeze-drying) or in desorbing gas from surfaces (e.g., the cleanup of microwave tubes during manufacture). Vacuum is also used in processes where the distance must be extended that a particle must travel before it collides with another, thereby permitting the particles to follow a collision-free course between source and target (e.g., in vacuum coating, particle accelerators, television picture tubes). Finally, vacuum is used in preparing clean surfaces, by reducing the number of molecular impacts per second. That decreases the chances of contamination (e.g., in clean-surface studies and preparation of pure, thin films).
In semiconductor wafer processing, vacuum is used during the thin-film deposition and etching operations, primarily to reduce contamination. The vacuum system of the invention, while described herein primarily in connection with a semiconductor wafer manufacturing operation, may be used in processes and research activities requiring any of the above uses of vacuum.
The realities of vacuum pump design are such that no one vacuum pump has been built that will operate in a pressure range from one atmosphere down to a “high vacuum” of 10−6 torr or lower, with a sufficient pumping speed to meet the requirements of some applications. Instead, to achieve a vacuum high enough for thin-film coating and other high vacuum applications, a pumping system that includes both a primary oil-sealed or dry pump and a secondary, high-vacuum molecular pump is used. The rotary oil-sealed or dry primary pump (or forepump or backing pump) “roughs” the process chamber down to a “low vacuum” pressure of about 0.1 torr, after which the secondary high-vacuum molecular pump and rotary pump are used in series to evacuate the process chamber down to high-vacuum levels needed for processing.
One reason for the use of two pump mechanisms in a high vacuum pumping system is that there are two physical regimes to consider in pumping a vacuum. In the low vacuum range, down to about 10−1 or 10−2 torr, air molecules interact. Under those conditions, air has viscous qualities and flows like a fluid, and can therefore be pumped using an oil-sealed or dry rotary pump.
At high vacuum pressures the molecules are independent of each other, resulting in “molecular flow.” A pump must work on each molecule. Under those conditions, “pumping” is really providing a point of no return (or low probability of return) in a system characterized by random molecular movement. A molecular pump provides such a point of no return.
Oil sealed pumps and dry rotary pumps are both used in vacuum pumping systems as backing pumps. In general, both types of pump rely on confining a volume of gas in a pumping chamber that is reduced in volume before exhausting on the high pressure side of the pump. Various geometric configurations are used in rotary vacuum pumps, including rotary vane pumps and interdigitated shapes rotating on parallel shafts.
Oil sealed rotary vane pumps comprise a single shaft driving a rotor with sliding vanes; the rotor and vanes rotate within an eccentric stator. The pump may have a single stage or may have two stages in series, with the larger first stage exhausting into a smaller secondary stage. The entire mechanism is immersed in oil for lubrication, sealing and cooling.
Known configurations of dry pumps include hook and claw, tongue and groove and screw geometries, and Roots pumps, among others. There is no oil in the dry pump mechanism; sealing is instead effected by close running clearances. While dry pumps are generally more difficult to manufacture and therefore more costly, they are preferred in the semiconductor manufacturing industry because they introduce fewer contaminants into the system, and because the oil in an oil-sealed pump tends to absorb corrosive process gasses and thereby degrade the pump.
Several techniques have evolved for pumping gas on a molecular level. Those include the diffusion pump that imparts momentum using a jet of vapor to move molecules in the vacuum chamber toward the exhaust. Gas capture pumps remove molecules by ion entrapment, freezing (cryo pumps) or by burying the gas under a constantly deposited film of metal.
Turbo pumps (or turbo molecular pumps) utilize a turbine-like rotor that accelerates molecules in the exhaust direction, increasing the probability that a molecule will move out of the chamber toward the backing pump. That technique has come to be used in applications where cleanliness is critical, because there is no problem with the back streaming of any materials used in the pumping mechanism; i.e., the pumping mechanism is dry.
None of the molecular pumps (diffusion, gas capture or turbo) is capable of efficient operation at atmospheric pressure. For that reason, as described above, a vacuum chamber is first evacuated to a roughing pressure of about 1 torr to 10−2 torr using a roughing pump, followed by further evacuation by a high-vacuum molecular pump. The molecular pump therefore typically has an exhaust pressure of about 1 torr to 10−2 torr throughout its duty cycle, although pumps capable of exhausting to a greater pressure are known in the art.
Abatement equipment must be used in many applications to condition exhaust in order to control the release of dangerous gasses into the atmosphere and to recapture materials used in the manufacturing process. One example of an abatement device is a scrubber, which removes material from an effluent by injecting a liquid or gas into the effluent. Available scrubbers include wet scrubbers and dry scrubbers.
In a wet scrubber, the process exhaust is forced into a spray chamber, where fine water particles dissolve gasses and entrain dust and particles, removing them from the gas stream. The dust- and solute-laden water is then treated to remove the captured material. The water may be recycled.
In a dry scrubber, a gas may be injected into the exhaust to chemically change hazardous gasses in the exhaust stream. Dry scrubbers may use a variety of techniques to remove the unwanted gasses, including thermal oxidation with or without additional fuel or oxidant, adsorption (hot or cold), and catalytic and plasma processes. Scrubbers are also known that comprise a dry stage feeding a wet stage.
Traps are also available that simply collect dust. Those traps may be atmospheric or low pressure. They may use a filter or a cyclone.
In a typical vacuum system for the manufacture of semiconductor wafers or for other reactive gas processes, a single turbo molecular pump and a single backing pump are provided in series to service a single process vacuum chamber, the turbo molecular pump being nearest the vacuum chamber. Four vacuum process chambers are typically provided on a single manufacturing tool. One or more abatement devices may be used for removing excess process gasses from the exhaust. If the abatement device is between the turbo and backing pump, currently one abatement unit is required per chamber. If the abatement device is atmospheric, i.e., downstream of the backing pumps, one abatement device may be shared among several chambers, provided that the unabated gasses are compatible, or provided that the abatement device acts sufficiently on the gas streams to render the gases compatible before they mix. One backing pump is currently required per chamber irrespective of chamber configuration to avoid pressure fluctuations in one chamber disturbing the pressures in the other chambers.
Various systems have been implemented for regulating pressure inside the vacuum chamber. In one such system, described in U.S. Pat. No. 6,419,455 to Rousseau et al., issued Jul. 16, 2002, the speeds of rotation of a turbo molecular pump and a backing pump are controlled simultaneously to achieve a predetermined pressure profile in the chamber.
Another system, described in European Patent Application EP 1014427 A2, published Jun. 28, 2000, uses a multiple-inlet secondary (low vacuum) pump in evacuating a plurality of process chambers. The secondary pump inlets may be connected to high vacuum pumps.
U.S. Pat. No. 5,873,942 to Park et al., issued Feb. 23, 1999, shows a vacuum exhaust system wherein several high-vacuum pumps are backed by a single low-vacuum pump. Gate valves are placed between the high vacuum pumps and the low vacuum pump. Abatement and incompatibility of the reactive gasses is not addressed.
A system described in U.S. Pat. No. 5,944,049 to Beyer et al., issued Aug. 31, 1999, utilizes a control valve placed on the exhaust side of a high vacuum pump. The control valve is used for regulating vacuum inside the process chamber.
Typical semiconductor wafer processing systems have several vacuum chambers with an independent vacuum pumping system for creating and maintaining a vacuum in the chamber. Process cycles within the chambers are run independently, with reactive gasses being admitted as required at various pressures.
The initial cost of installing such a system is high, due in part to the many duplicate components such as abatement devices and backing pumps. For similar reasons, maintenance costs for such a system are high and the system occupies a large amount of space.
A semiconductor wafer processing system typically has several successive steps in which a substrate is sequentially exposed to a series of reactive gasses. Those steps take place in separate process vacuum chambers, and involve different and often incompatible reactive gasses. In those cases, the four or more vacuum chambers on a single semiconductor manufacturing tool may emit exhaust gasses that may not be mixed unconditionally in a vacuum exhaust system.
Traps for treating multiple impurities are available. For example, a system described in U.S. Pat. No. 4,784,837 to Kitayama et al., issued Nov. 15, 1988, utilizes an absorbent and process for removing multiple materials from semiconductor processing gasses. Similarly, U.S. Pat. No. 5,904,757 to Hayashi et al., issued May 18, 1999, discloses a trap for removing several reaction byproducts from the exhaust of a single semiconductor processing chamber.
There is therefore presently a need to provide a vacuum exhaust apparatus and method that uses a single low-vacuum pump for backing several high-vacuum pumps evacuating several process chambers. Particularly, the apparatus and method should be suitable for use with two or more process chambers having incompatible process materials in their exhaust gasses. To the inventors' knowledge, there is currently no such technique available.